Reversible switching from fluorescence to room temperature phosphorescence amplified by exciton-vibration coupling through pressure-induced tiny packing changes

Investigating the impact of exciton–vibration coupling (EC) of molecular aggregates on regulating the excited-state dynamics and controlling room temperature phosphorescence (RTP) emissions is crucial and challenging. We designed and synthesized ArBFO molecules and cultured two crystals with similar molecular packing and completely different luminescent mechanisms from B-form fluorescence to G-form RTP. The mechanism study combining measurement of photophysical properties, time-resolved fluorescence analysis, X-ray diffraction analysis, and theoretical calculations shows that tiny changes in molecular stacking amplify the EC value from B-form to G-form H-aggregates. The larger EC value accelerates the ISC process and suppresses the radiative singlet decay. Meanwhile, the stronger intermolecular interaction restricts non-radiative transitions. All of these facilitate green RTP emission in G-form aggregates. When treated with pressure–heating cycles, the transformation between B-form and G-form aggregates leads to a reversible blue fluorescence/green RTP switch with good reproducibility and photostability. Moreover, their potential in multi-level information encryption and anti-counterfeiting application has been well demonstrated. The results of this research deepen the understanding of the effect of aggregation on the luminescence mechanism and provide a new design guidance for developing smart materials with good performance.


Preparation of the crystal
Two kinds of polymorph crystals with high quality and homogeneity were obtained in high throughput under different growth conditions.The color of these crystals differs from green (Bform) to orange (G-form).B-form single crystals with sizes about 3 mm × 1 mm were prepared by the solvent diffusion method at the liquid-liquid interface between hexane and the dichloromethane solution in the 5 mL glass sample at room temperature.G-form single crystals with sizes about 3 mm × 1 mm were prepared by the solvent diffusion method at the liquid-liquid interface between cyclohexane and the dichloromethane solution in the 5 mL glass sample at room temperature. From the excitation spectra and diffuse reflectance absorption spectra, the excitation spectra of the two crystals are relatively consistent with their respective diffuse reflectance absorption spectra. We collected the temperature dependent photoluminescence spectra of B-form and G-form crystals.As shown in Figure S4a, at room temperature, the maximum emission peak is displayed at 460 nm, and its intensity increases with decreasing temperature.As shown in Figure S4b, as the temperature decreases, the PL spectrum of the green crystal remains unchanged, with a maximum value of around 499 nm.At the same time, the PL intensity at 77K is significantly enhanced by about 6 times compared to that at 298K. The main emission peak of the steady-state spectrum at 298 K is 499 nm.And the main emission peak of phosphorescence at 77 K is approximately 510 nm, which means the maximum emission wavelength undergoes a slight redshift at low temperature.In addition, the PL and phosphorescence spectra both at 77 K in Figure 2f are different significantly.These results all indicated that the delayed PL at 298 K might not be entirely attributed to phosphorescence.To verify this, the lifetime measurement of different wavelengths in PL spectrum of G-form crystal at 298 K and 77 K were performed.At 298 K, the lifetime of the emission wavelength at 460 nm is 3.52 μs, and at 77 K, the lifetime is decreased to 1.98 μs, exhibiting a trend of decreasing lifetime with the decrease of temperature.This is a typical feature of thermally activated delayed fluorescence (TADF).Therefore, a small amount of TADF component at 460 nm might exist in the spectrum of G-form.

Photophysical properties
Table S1.Summary of the single crystals data of B-form and G-form. Encouraged by the highly sensitive PCF properties with high emission, the ArBFO B-form power aqueous inks were prepared.The preparation procedure is as follows: after the prepared poly (vinyl alcohol) PVA aqueous solution (0.05 g• mL -1 ) was prepared via dissolving 1 g of PVA in 20 mL of DI water at 80 °C for 2 h.Disperse 10 mg ArBFO powder into 1 mL of the above PVA aqueous solution.The solution was then stirred for 10 min to form a homogeneous ArBFO ink.Write "0" on the filter paper using ArBFO ink, and after crushing, the emission color changes from blue to green. Note that the ArBFO microcrystals encapsulated in the PVA medium still maintain the sensitive PCF behaviors after completely drying in air.Drop ArBFO ink onto filter paper to form a uniform film, and after rolling, the emission color changes from blue to green.More impressively, ArBFO ink can be drawn on different flexible substrates, including, fabric, tinfoil, and so on, which shows great potential applications in the field of haptic sensors and wearable flexible anticounterfeiting devices. The preparation methods for dye Ⅰ and dye Ⅱ water-based inks are the same as those for ArBFO water-based inks.

Figure S5 .
Figure S5.Normalized diffuse reflectance absorption and excitation spectra of B-form (a) and G-form crystals (b).

Figure S6 .
Figure S6.Steady-state spectra of B-form crystals (a) and G-form crystals (b).

Figure S7 .
Figure S7.Temperature-dependent transient decay spectra of G-form crystals.

Figure S8 .
Figure S8.PL decay curves of B-form crystals measured at 520 nm in 77K.

Figure S9 .
Figure S9.Lifetime of G-form with an emission wavelength of 460nm at 298 K and 77 K

Figure S10 .
Figure S10.The overlap of (a) B-form and (b) and G-form crystal.

Figure S11 .
Figure S11.Exciton coupling values of (a) B-form (b) and G-form crystal.

Figure S12 .
Figure S12.Setting of QM/MM model for (a) B-form (b) and G-form crystal.

Figure S13 .
Figure S13.(a) Representative photos of the quantitative pressure test by using a digital thrust meter.(b) Cyclic switch of the wavelength and intensity during the pressure -heating process.(c) PXRD patterns of power upon pressure.The simulated results based on the single-crystal data are also included.

Figure S14 .
Figure S14.Emission spectra of two types of powder crystals during pressure-heating cycle.

Figure S15 .
Figure S15.The chemical structural formulas of dye I , dye II and the state of dye I, dye II and B-form under daylight.

Figure S16 .
Figure S16.The relative PL intensity of ArBFO power for 4 days.

Figure S17 .
Figure S17.Afterglow photos of ArBFO at different times at 77 K